Various electro-optical systems have been developed for reading optical indicia, such as bar codes. A bar code is a coded pattern of graphical indicia comprised of a series of bars and spaces of varying widths, the bars and spaces having differing light reflecting characteristics. The pattern of the bars and spaces encode information. Bar code may be one dimensional (e.g., UPC bar code) or two dimensional (e.g., DataMatrix bar code). Systems that read, that is, image and decode bar codes employing imaging camera systems are typically referred to as imaging-based bar code readers or bar code scanners.
Imaging-based bar code readers may be portable or stationary. A portable bar code reader is one that is adapted to be held in a user's hand and moved with respect to a target indicia, such as a target bar code, to be read, that is, imaged and decoded. Stationary bar code readers are mounted in a fixed position, for example, relative to a point-of-sales counter. Target objects, e.g., a product package that includes a target bar code, are moved or swiped past one of the one or more transparent windows and thereby pass within a field of view of the stationary bar code readers. The bar code reader typically provides an audible and/or visual signal to indicate the target bar code has been successfully imaged and decoded.
A typical example where a stationary imaging-based bar code reader would be utilized includes a point of sale counter/cash register where customers pay for their purchases. The reader is typically enclosed in a housing that is installed in the counter and normally includes a vertically oriented transparent window and/or a horizontally oriented transparent window, either of which may be used for reading the target bar code affixed to the target object, i.e., the product or product packaging for the product having the target bar code imprinted or affixed to it. The sales person (or customer in the case of self-service check out) sequentially presents each target object's bar code either to the vertically oriented window or the horizontally oriented window, whichever is more convenient given the specific size and shape of the target object and the position of the bar code on the target object.
A stationary imaging-based bar code reader that has a plurality of imaging cameras can be referred to as a multi-camera imaging-based scanner or bar code reader. In a multi-camera imaging reader, each camera system typically is positioned behind one of the plurality of transparent windows such that it has a different field of view from every other camera system. While the fields of view may overlap to some degree, the effective or total field of view of the reader is increased by adding additional camera systems. Hence, the desirability of multicamera readers as compared to single camera readers which have a smaller effective field of view and require presentation of a target bar code to the reader in a very limited orientation to obtain a successful, decodable image, that is, an image of the target bar code that is decodable.
The camera systems of a multi-camera imaging reader may be positioned within the housing and with respect to the transparent windows such that when a target object is presented to the housing for reading the target bar code on the target object, the target object is imaged by the plurality of imaging camera systems, each camera providing a different image of the target object. U.S. patent application Ser. No. 11/862,568 filed Sep. 27, 2007 entitled ‘Multiple Camera Imaging Based Bar Code Reader’ is assigned to the assignee of the present invention and is incorporated herein by reference. U.S. patent application Ser No. 12/112,275 entitled “Bar Code Reader having multiple Cameras” filed Apr. 30, 2008 is assigned to the assignee of the present invention and is also incorporated herein by reference. U.S. Pat. No. 5,717,195 to Feng et al concerns an “Imaging Based Slot Datform Reader” having a mirror, camera assembly with photosensor array and a illumination system. The disclosure of that patent is incorporated herein by reference.
Light emitting diodes (LEDs) have been used to illuminate a bar code reader field of view. An LED concentrates much of its energy within its field of view (FOV). For instance, most LEDs specify a FOV angle where 50% of the light emitted for it is contained. FIG. 6 is an example of a data sheet for a representative light emitting diode wherein fifty percent of the light intensity is contained in a cone of light which subtends an angle of 60 degrees. The remainder of the light may be dispersed in a field of 180 degrees. Lenses are sometimes placed in front of the LEDs in order to capture as much of the light being emitted from them as possible. However, even a lens (or series of lenses) may not capture all of the light into the desired FOV. The light beyond the desired FOV is stray light and may end up in the undesirable places such as in the eyes of a user or customer who happens to be near the bar code reader.